2018年, 第21卷, 第4期　
刊出日期：2018-12-25

  

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  Zircon U-Pb geochronology, geochemistry and geological significance of Xiarihamu granodiorite in Eastern Kunlun orogenic belt, Qinghai

  YANG Baorong, MA Zhongxian, ZHANG Libin, WANG Xiaoyun, YAN Zhengping

  Global Geology. 2018, 21(4): 209-220. https://doi.org/10.3969/j.issn.1673-9736.2018.04.01

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  Zircon U-Pb geochronology and geochemistry of the granodiorites in Xiarihamu Cu-Ni mining area are studied to determine the formation age, genesis and tectonic setting. The results show that the granodiorites formed in the Middle Triassic with weighted average age of 242±1 Ma. The granodiorites comprise of SiO₂ of 61.34%-62.54%, Al₂O₃ of 16.46%-16.87%, MgO of 2.37%-2.66%, (Na₂O+K₂O) of 5.39%-5.74% with the Na₂O/K₂O of 1.46-1.88, Mg^# of 47.03-48.04, and show metaluminous characteristics with A/CNK of 0.94-0.96, belonging to calc-alkaline I-type granites geochemically. The light and heavy rare earth elements in the rocks are distinctly fractionated and have "right-inclined" distribution patterns with weak negative Eu anomalies (δEu=0.84-0.91). Moreover, these samples show enrichments in LILEs (e.g., Rb, Ba, K and Sr), especially in Sr[(343-452)×10^-6], and obvious depletions in HFSEs (e.g., Nb, Ta, Ti and P), indicating geochemical characteristics similar to subduction-related magmatic rocks. According to the regional tectonic evolution, the Xiarihamu granodiorites were emplaced under an Andean-type active continental margin environment in the early Indosinian, possessing a crust-mantle mixing origin.
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  U-Pb geochronology and geochemical characteristics of Liudaogou granodiorite in Linjiang, Jilin Province

  ZHENG Yuanhao, ZHAO Fengchen, WANG Li, LIU Jinlong, WU Qiong, TIAN Lidan, YIN Yue

  Global Geology. 2018, 21(4): 221-231. https://doi.org/10.3969/j.issn.1673-9736.2018.04.02

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  The Liudaogou area underwent multiple large-scale tectonic movements since Archean. Little studies have been carried out on the age of Mesozoic magmatism and petrogenesis in the area. Zircon U-Pb geochronology and geochemistry of the Liudaogougranodiorite were undertaken in this study to determine the forming-age of the granodiorite and to understand its petrogenesis and tectonic background of diagenesis. Zircon in the rock samples show good idiomorphic and obvious rhythmic growth girdle zone, belonging to typical magmatic zircon. Zircon LA-ICP-MS U-Pb dating results show that the emplacement of rock mass occurred in late Early Cretaceous (125±1 Ma, MSWD=0.53). The geochemical data show that the rock belongs to quasi-aluminous granite of type I which has the properties of high potassium calc-alkaline adakite, with A/CNK ratio of 0.82-0.98 (<1.1). The rock shows obvious enrichment of LILE and LREE but depletion of HFSE, and is characterized by high Sr (374.9×10^-6-777.6×10^-6) and low Y (8.84×10^-6-12.4×10^-6). The samples are rich in Si, and the K₂O/Na₂O ratio (0.99-1.30) is very high, indicating that magma might have originated from melting upper crust. The high content of Mg^# (49.68-56.92) in the samples implies that mantle-derived materials might have involved in magma. It can be concluded that in the process of delamination, the lower crust sank, and was heated by asthenosphere mantle, and partially melted. The primitive magma should be the mixture of the mantle-derived magma and the partially melting lower crust. Integrating research results on the regional contemporary igneous rock, it can be concluded that the rocks were formed in an active continental margin environment related to the subduction of the Paleo-Pacific Plate to the Eurasian Plate.
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  Sedimentary facies in Early Cretaceous strata,Shuangyashan Basin

  HE Yanhui, ZHU Jianwei, ZHOU Yongheng

  Global Geology. 2018, 21(4): 232-236,251. https://doi.org/10.3969/j.issn.1673-9736.2018.04.03

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  The sedimentary facies of Chengzihe and Muling formations of Early Cretaceous coal-bearing strata in Shuangyashan Basin were studied via petrography, grain-size analysis and borehole data analysis. Based on the sedimentary characteristics and temporal and spatial distribution of the sedimentary facies, sedimentary facies of the Early Cretaceous Chengzihe and Muling formations were mapped. The results suggest that fan delta and lacustrine are the major facies developed, which include fan delta plain, shallow lake and multi-layered marsh coal. For the period from the Chengzihe to the Muling Formation, the sedimentary basin expanded with lake basin migrating to the northeast. The period of the Chengzihe Formation represents a period of growth for delta plain deposits and coal accumulation, whereas Muling Formation represents the development of lacustrine deposits, consisting of mudstone and a few thin coal seams.
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  Feasibility study on replacing chemical uranium assays with uranium equivalent grades at Husab Uranium Project, Namibia

  CAO Yun, LUO Yi, LI Zheng

  Global Geology. 2018, 21(4): 237-244. https://doi.org/10.3969/j.issn.1673-9736.2018.04.04

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  Formerly, uranium exploration at the Husab Uranium Project was based on sample chemical uranium assays with little uranium equivalent data (eU) obtained from γ probing. The high cost of performing chemical assays has a significant impact on the overall cost of uranium exploration. Between 2015 and 2016, the Resource Upgrade Project Zone 2 was undertaken at the Husab Project, which identified the reliability and feasibility of γ probing through comparison of chemical uranium assays with uranium equivalent grades. This project analyzed the sources of errors and proposed a method to correct them. Uranium equivalent data from gamma probing can be used to replace the chemical assays to reduce the cost and time, therefore increasing the efficiency for uranium exploration at the Husab Uranium Project.
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  Inversion of gravity gradient data based on spatial gradient weighting

  JIANG Dandan, YU Ping, LIN Song, GAO Xiuhe

  Global Geology. 2018, 21(4): 245-251. https://doi.org/10.3969/j.issn.1673-9736.2018.04.05

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  Compared with traditional gravity measurement data, gravity gradient tensor data contain more high frequency information, which can be used to understand the earth's interior structure, mineral resources distribution etc. In this study, the authors present an algorithm for inverting gravity gradiometer data to recover the three-dimensional (3-D) distributions of density. Spatial gradient weighting was used to constrain the extent of the body horizontally and vertically. A more accurate inversion result can be obtained by combining the prior information into the weighting function and applying it in inversion. This method was tested on synthetic models and the inverted results showed that the resolution was significantly improved. Moreover, the algorithm was applied to the inversion of empirical data from a salt dome located in Texas, USA, which demonstrated the validity of the proposed method.
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  Combination of structure tensor and tilt angle in the edge detection of potential field data

  LIN Song, LI Lili, SHI Jianan

  Global Geology. 2018, 21(4): 252-259. https://doi.org/10.3969/j.issn.1673-9736.2018.04.06

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  Edge detection is a commonly requested task in the interpretation of potential field data. Different methods have different results for varied depths and shapes of geological bodies. In this paper, we propose using the combination of structure tensor and tilt angle to detect the edges of the sources, which can display the edges of shallow and deep bodies simultaneously. Through tests on synthetic potential field data, it is obvious that the proposed edge detection methods can display the sources edges more clearly and precisely, compared with other commonly used methods. The application on real potential field data shows similar result, obtaining the edges of layers and faults clearly. In addition, another advantage of the new method is its insensitivity to noise.
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  Advanced derivative ratio filters for edge detection of potential field data

  MING Yanbo, GAO Tong, MA Guoqing

  Global Geology. 2018, 21(4): 260-266. https://doi.org/10.3969/j.issn.1673-9736.2018.04.07

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  Edge location is an important information of the source, and can be obtained by the potential field data. Most edge detection methods of potential field data are the functions of horizontal and vertical derivatives. The authors provide a new strategy to establish edge detection filters that can improve the resolution to identify small bodies, which use the ratio functions of different-order derivatives to recognize the edges of the sources. The new filter is named as advanced derivative ratio (ADR) filter and balanced outputs can be produced for different forms of ADR filters. The ADR filters are tested on synthetic data and real potential field data. The advantage of the ADR filters is that they can detect the edges of the causative sources more precisely and clearly, and the model testing results show that the resolution of ADR filters is higher than other existing filters. The ADR filters were applied to real data, with more subtle details obtained.